The disclosures of Japanese Patent Applications No. 2000-350167, No. 2001-039113 and No. 2001-070706, each including the specification, drawings and abstract, are each incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a reducing agent. The invention also relates to an emission control system that uses the reducing agent for removing or reducing NOx contained in an exhaust gas emitted from an internal combustion engine.
2. Description of Related Art
NOx catalysts of selective-reduction type are widely used in emission control systems arranged to remove or reduce NOx in exhaust gas emitted from an internal combustion engine (e.g., a diesel engine or a lean-burn gasoline engine) that is operable at a lean air/fuel ratio (higher than the stoichiometric value). Such NOx catalysts of selective-reduction type use a reducing agent for reducing or decomposing harmful NOx in the exhaust emission, under an oxygen-rich atmosphere.
The NOx catalyst of selective-reduction type as described above commonly employs hydrocarbon as the reducing agent. However, it has been proposed to use other kinds of reducing agents, such as urea and other ammonia-derived reducing agents. JP-A-7-323216 discloses an exemplary reducing-agent supply device using solid urea as the reducing agent.
In the reducing-agent supply device disclosed in JP-A-7-323216, a fuel-burning layer of catalyst is disposed in a portion of an exhaust passage, which is upstream of a denitration device. When a combustion device (engine) is started, fuel is sprayed for combustion in a portion of the exhaust passage upstream of the fuel-burning catalyst layer. The supply of the fuel is terminated when the temperature of the denitration device has reached a predetermined value. Then, the reducing-agent supply device introduces a solid or liquid reducing agent into a portion of the exhaust passage upstream of the fuel-burning catalyst layer. The above-identified publication teaches the use of a powdered reducing agent, such as melamine, urea and cyanuric acid.
With the powdered reducing agent sprayed over the fuel-burning catalyst layer, the reducing agent is vaporized for denitration reaction with NOx in the exhaust gas, whereby NOx in the exhaust gas is converted into nitrogen.
The denitration device for denitration of the exhaust gas as disclosed in the publication JP-A-7-323216 uses a solid reducing agent stored in a storage tank. Where solid urea alone is used as the solid reducing material, the solid urea agglomerates as a result of absorption of an aqueous component during the storage in the tank, and therefore suffers from a low degree of mobility and difficult control of the amount of supply from the tank.
In the case where a catalytic device for converting NH3 into NO is present in the exhaust passage, the introduction of a reducing agent such as solid urea to a portion of the passage upstream of the catalytic device causes oxidization of the reducing agent due to its flow through the catalytic device. As a result, the reducing agent is not able to reduce or remove NOx in the exhaust gas at the catalyst of the denitration device disposed downstream of the catalytic device. Accordingly, the portion of the exhaust passage into which the reducing agent is introduced must be determined depending upon the specific position of the catalytic device in the exhaust passage.
It is therefore a first object of the invention to provide a solid reducing material that does not agglomerate during storage and supply.
A second object of this invention is to provide an emission control system including a reducing-agent supply device, which permits efficient introduction of a solid reducing agent.
A third object of the invention is to provide an emission control system including a reducing-agent supply device, which uses a reducing agent that does not agglomerate during storage and supply, and which is capable of introducing the reducing agent into a portion of an exhaust system of an internal combustion engine, such that the introduction of the reducing agent into that portion of the exhaust system does not deteriorate the exhaust emission of the engine.
To accomplish the first object indicated above and/or other objects, there is provided according to a first aspect of the invention a reducing agent comprising a mixture of an ammonia-derived solid reducing material and a water-insoluble liquid in which the solid reducing agent is immersed or dispersed.
The ammonia-derived solid reducing material may be urea, biuret or ammonium carbamate, for example.
The solid reducing material may be in a powdered or pelletized form. The particle or pellet size of the solid reducing material is desirably selected for easy handling by an associated system or device, and is preferably not larger than about 3.0 mm.
The water-insoluble liquid may be selected from a wide variety of mineral oils and plant oils. Examples of the mineral oils include light oil, kerosene, gasoline and silicone oil. Examples of the plant oils include rapeseed oil, coconut oil and eucalyptus oil. At least one of the mineral and plant oils may be used as the water-insoluble liquid. A mixture of two or more mineral and/or plant oils may be used as the water-insoluble liquid, as long as the mixture is possible.
Any water-insoluble liquid may be used as long as the water-insoluble liquid does not dissolve the solid reducing material within the mass of the liquid, and does not have an adverse influence on a catalyst to which the solid reducing material is to be applied. Where the reducing material is used for a catalyst for purifying exhaust gases emitted from an internal combustion engine, a fuel, such as gasoline or light oil used by the engine, is preferably used as the water-insoluble liquid for easy handling.
The temperature of the mixture is preferably lower than an upper limit above which the solid reducing material is fused or melted. This limitation of the temperature is desirable to assure easy control of introduction of the mixture and to effectively introduce only the solid reducing material.
Described in detail, the temperature of the mixture during storage or supply is lower than the lower one of (i) the boiling point of the water-insoluble liquid and (ii) the fusing or melting point of the solid reducing agent. Where the mixture includes solid urea as the solid reducing material and light oil as the water-insoluble liquid, the boiling point of the light oils ranges from about 250xc2x0 C. to about 350xc2x0 C. while the fusing point of the solid urea is about 130xc2x0 C., so that the temperature of the mixture is preferably held below the fusing point of the solid urea.
The water-insoluble liquid preferably has a lower specific gravity than the solid reducing material, so that the solid reducing material does not float on, or entirely exist adjacent to the surface of, a mass of the water-insoluble liquid, but is distributed or dispersed within or throughout the mass of the water-insoluble liquid, in order to prevent agglomeration of the solid reducing material into a large solid mass or to improve the ease of handling of the reducing material.
A solid reducing agent stored in a suitable storage device may be fed by any suitable feeding device into another storage device provided with a suitable heater for liquefying the solid reducing agent, so that the resulting liquid reducing agent is introduced by a suitable introducing device into a portion of an exhaust passage, which is upstream of an NOx catalyst of selective-reduction type.
The internal combustion engine for which the reducing agent is used according to the invention may be a lean-burn gasoline or diesel engine of direct fuel injection type, for example.
The NOx catalyst of selective-reduction type may include a substrate formed of zeolite and an ion-exchanged transition metal carried on the substrate. The NOx catalyst may alternatively include a substrate formed of zeolite or alumina and a noble metal carried by the substrate.
The solid reducing agent may be introduced at a desired rate into an exhaust passage by a suitable supplier such as an injector, which is controlled by a suitable control device, for example, an electronic control unit (ECU) generally used for controlling an internal combustion engine. The solid reducing agent to be injected into the exhaust passage need not be liquefied or gasified (vaporized), but may be fluidized while remaining in a solid state. Alternatively, the solid reducing agent in a powdered form may be introduced into the exhaust passage.
In the first aspect of the invention, the solid reducing material is stored as a mixture with the water-insoluble liquid, as described above, so that the solid reducing material does not absorb an aqueous component contained in the air, and the particles or pellets of the solid reducing material do not agglomerate into a large solid mass. In addition, the reducing agent has a high degree of mobility for easy introduction into the exhaust passage, in the presence of the water-insoluble liquid.
To accomplish the second object and/or other object(s), there is provided according to a second aspect of the invention a reducing-agent supply device for an internal combustion engine, for supplying a denitration catalyst with a reducing agent. The reducing agent comprises a mixture of an ammonia-derived solid reducing material and a water-insoluble liquid. The reducing-agent supply device is operable to introduce the reducing agent into a portion of an exhaust passage of the internal combustion engine which is upstream of the denitration catalyst. The denitration catalyst is arranged to induce a reaction between NOx and ammonia (NH3), for promoting the conversion of NOx into nitrogen (N2). For example, the denitration catalyst includes a substrate formed of titania, and molybdenum oxide or vanadium oxide carried on the titania substrate.
When the exhaust passage of the internal combustion engine has only the denitration catalyst, the reducing agent is introduced into a portion of the exhaust passage between the internal combustion engine and the denitration catalyst. In this case, there is induced a reaction between the reducing agent and NOx contained in the exhaust gas, so that NOx is converted into harmless nitrogen.
When a catalyst for converting NH3 into NO is disposed in a portion of the exhaust passage between the internal combustion engine and the denitration catalyst, the reducing agent is introduced into a portion of the exhaust passage between the above-indicated converting catalyst and the denitration catalyst. In this case, the reducing agent is applied to the downstream denitration catalyst, without oxidization by the converting catalyst, so that a denitration reaction is induced at the denitration catalyst, between the reducing agent and NOx in the exhaust gas.
The catalyst for converting NH3 into NO is disposed in a relatively upstream portion of the exhaust passage. The catalyst may be an oxidizing catalyst, a three-way catalyst, an NOx catalyst of occlusion-reduction type, an NOx catalyst of selective-reduction type, a DPR, DPNR or an adsorption catalyst.
The oxidizing catalyst is arranged to oxidize HC and CO in the exhaust gas, into H2O and CO2. The three-way catalyst is arranged to perform the function of the oxidizing catalyst, and is at the same time capable of reducing NOx in the exhaust gas when the exhaust gas is produced as a result of combustion of an air-fuel mixture having the stoichiometric air/fuel ratio. Thus, the three-way catalyst is capable of converting the three harmful gas components into harmless CO2, H2O and N2.
The NOx catalyst of occlusion-reduction type is arranged to absorb NOx when the exhaust gas has a lean air/fuel ratio, and releases the absorbed NOx for reduction into N2 when the oxygen concentration of the exhaust gas is lowered.
The NOx catalyst of selective-reduction type is arranged to reduce or decompose NOx in the presence of HC in the oxygen-rich atmosphere.
The DPR (Diesel Particulate Reactor) or DPNR (Diesel Particulate NOx Reduction) is a particulate filter carrying an active-oxygen releasing agent, for oxidizing particulate deposited on the particulate filter. Further, the DPR or DPNR is capable of absorbing NOx when the exhaust gas has a lean air/fuel ratio, and releasing the absorbed NOx for reduction into N2 when the oxygen concentration of the exhaust gas is lowered.
To accomplish the above-indicated third object and/or other object(s), there is provided according to a third aspect of the invention an emission control system for an internal combustion engine. The system includes (a) a denitration catalyst disposed in an exhaust passage of the internal combustion engine, to reduce or decompose NOx, and (b) a reducing-agent supply device disposed near the exhaust passage. The reducing-agent supply device is operable to introduce a controlled amount of a reducing agent into the exhaust passage for use with the denitration catalyst. The reducing agent comprises a mixture of an ammonia-derived solid reducing material and a water-insoluble liquid. In this system, the reducing-agent supply device introduces the reducing agent into the exhaust passage when a first purification percentage representing a capability of the denitration catalyst to remove the water-insoluble liquid is greater than a threshold value.
Preferably, the introduction of the reducing agent into the exhaust passage is initiated when a second purification percentage representing a capability of the denitration catalyst to remove HC and a third purification percentage representing a capability of the denitration catalyst to remove NOx, in addition to the first purification percentage, are greater than respective threshold values. The purification percentage of each of the water-insoluble liquid, HC, and NOx may be determined on the basis of the detected temperature of the exhaust gas or denitration catalyst.
The denitration catalyst is arranged to induce a reaction between NOx and ammonia (NH3), for promoting the conversion of NOx into nitrogen (N2). For instance, the denitration catalyst includes a substrate formed of titania, and a molybdenum oxide or vanadium oxide carried on the titania substrate.
The arrangement can prevent the introduction of the reducing agent before the temperature of the exhaust gas has reached a value above which the water-insoluble liquid can be removed from the exhaust gas, so that the water-insoluble liquid of the reducing agent does not pass through the denitration catalyst into the atmosphere. Where the introduction of the reducing agent is initiated when the purification percentages of the water-insoluble liquid, HC and NOx have all exceeded the predetermined values, the passage of not only the water-insoluble liquid, but also HC and NOx into the atmosphere, are prevented.
According to the principle of this invention described above, the solid reducing material is stored as a mixture with the water-insoluble liquid, so that the solid reducing material does not absorb an aqueous component contained in the air, and the particles or pellets of the solid reducing agent do not agglomerate into a large solid mass.
In addition, the water-insoluble liquid functions to improve the mobility of the reducing agent including the solid reducing material, so that the solid reducing material, such as solid urea, can be easily introduced into the exhaust passage. Further, the reducing agent can be introduced in an appropriate condition of the exhaust gas or denitration catalyst, so as to assure efficient removal of NOx from the exhaust gas.